DC to DC converters provide stable regulated output voltages by conversion of DC input voltages to power processors, ASICS, memory, and other circuitry. Various types and forms of DC to DC converters have been developed, including buck converters to provide an output voltage lower than the input voltage, as well as boost converters capable of providing output voltages higher than the input voltage. Buck-boost converters offer the capability of providing a regulated output voltage at a level that can be greater than or less than the input voltage. One form of buck-boost converter is known as a cascaded buck boost converter created by cascading a buck power stage followed by a boost power stage. Cascaded buck boost converters offer higher efficiency and occupy less space than classic buck-boost converter topologies, where the converter operates in a pure buck mode when the input voltage is above the desired output voltage, and operate in pure boost mode when the input voltage is less than the desired output voltage. When the input and output voltages are approximately equal, a four switch buck-boost mode can be used with diagonal switches of an H-bridge configuration being turned on in alternating fashion, but this classical buck-boost mode involves switching all four converter switches in a given cycle, and is therefore inefficient.
So called “buck or boost” operation can be used when the input and output voltages are close to one another, in which a certain number of “buck” cycles are followed by a number of “boost” cycles, etc., in order to improve efficiency over traditional H-bridge type buck-boost operation. However, the number of consecutive buck or boost cycles determines the width of the buck or boost operating band or range, and it is desirable to operate in one mode for only a small number of cycles before switching to the other mode to keep the band narrow and thus reduce undesirable low frequency AC ripple at the output. As the number of consecutive buck or boost cycles is reduced, however, the number of transitions between these modes increases. Moreover, transitioning between buck mode and boost mode when using slope compensation for peak current mode control or valley current mode control causes undesirable output voltage ripple due to wide PWM pulse widths during transition. Thus, reducing the width of the “buck or boost” band will reduce the undesirable low frequency AC ripple, but large pulse widths while transitioning can result in excessive output ripple in the output. For example, a slope compensation ramp is added to the current sense signal in peak current mode control when the pulse width modulation duty cycle is greater than 50%, and the error amplifier output is at a level artificially higher than the peak inductor current. Transitioning from buck mode to boost mode for peak current mode control therefore involves mode change while the slope compensation is large, and changing to boost mode changes the duty cycle to less than 50% in which the slope compensation is small. This results in an excessively long pulse width at the transition, causing output voltage ripple. The same problem exists for valley current control mode operation. Consequently a need remains for improved cascaded buck boost DC to DC converters and control apparatus by which buck or boost transition is smooth and the buck or boost window or band is narrow.